1. Field of the Invention
This invention relates to artificial horizons of the type used to project a line of light in front of the pilot of an aircraft, and more particularly concerns projection apparatus of an artificial horizon having extended length and viewable by peripheral vision of a pilot to indicate roll and pitch of the aircraft.
2. Description of the Prior Art
Conventional aircraft instruments include a device which indicates the orientation of the aircraft relative to a horizon. The instrument is located in the cockpit where the pilot can inspect periodically the relatively small image representing a horizon to determine whether the aircraft is in a condition of roll with respect to its longitudinal axis, or pitch with respect to a lateral axis. This requires the pilot to use central vision which encompasses a solid angle of about 3 degrees directly in front of the pilot's eyes.
Central vision regards an instrument such as an artificial horizon as presenting a symbol which requires decoding and interpretation before any sense of spatial positioning can be obtained from it. Peripheral vision on the other hand, is used naturally for sensing motion in everyday situations, and is accomplished by means of dedicated "hardwired" circuits which recognize line-like features in the peripheral visual field, and convey details about their movements to the centers of the brain responsible for motion perception. Thus any instrument which provides information about movement of the pilot and aircraft by means of his peripheral vision will make use of these dedicated circuits, and frees the pilot's conscious thinking from the business of constantly interpreting symbols.
Since it is not practical to extend the size of the existing artificial horizon so that it can be viewed and perceived by the peripheral vision of the viewer, means must be devised to overlay the normal instrument panel with visible information which can be perceived at the periphery of the vision field. The most practical means known at present is to shine a line or bar of light from a projector onto a standard instrument panel, with means provided for causing the bar of light to seem to move relative to the aircraft to indicate the position of the aircraft with respect to the actual horizon.
One prior apparatus provides a line of light or a light bar in front of the pilot for observation by peripheral vision as disclosed in U.S. Pat. No. 4,083,239. A light source is mounted in a housing. One part of the housing is made to tilt and another part rotates thereby moving optical elements which transmit a light bar in front of the pilot. Movement of the parts is related to the aircraft's gyro platform so that the light oar gives a display representing the true horizon at shallow bank angles. Although this apparatus is acceptable in some applications, it is not practical in all aircraft primarily because the structure is quite bulky and will not fit readily into the cockpit of existing aircraft. In addition the apparatus does not project a true artificial horizon inasmuch as the pitch axis rolls as the aircraft rolls, that is, the pitch axis is not free floating within the roll gimbal and therefore the projector cannot display pitch information when the aircraft is rolled 90.degree.. Other systems which have been proposed for generating an artificial horizon viewable by peripheral vision are described in U.S. Pat. Nos. 4,397,555 and 4,616,226. These systems have the disadvantage that they are quite complicated in structure circuitry and adjustment, and are expensive to manufacture.